Sight glass liquid level gages have a vertical window that allows the level of a liquid in a vessel to be determined. Level gages are selectively isolated from vessel contents by valves to enable removal of a level gage for cleaning or repair without dumping the contents of the vessel. Liquid level gage valves are used in pairs, one on each end of a level gage. Typically, liquid level gage valves have a safety ball check that functions to quickly seat and prevent the vessel from emptying in the event of a break of the gage glass. Gage glass breakage will result in a sudden drop in pressure across the gage valve, which seats the safety ball. The valve may then be manually closed and the gage repaired. In a typical gage valve, closing of the valve causes a small extension on the bottom end of the stem to dislodge the seated check ball from the seat, thereby reestablishing communication between the vessel and the gage. The valve may then be fully opened to its normal position. Another feature common to liquid level gage valves is an offset of the gage connection. The offset feature facilitates cleaning of the gage glass by allowing a brush or swab to pass through the valve body and into the gage glass chamber once the vent and drain plugs are removed.
A problem common to standard valves is the fact that internals of the valves are in constant contact with the liquid of the vessel. The liquid can be very corrosive and may contain suspended particles. Since the valves are normally open, swelling associated with corrosion and/or suspended particles tend to collect and pack in the stem thread area, which can cause the valve to “freeze up” and become impossible to close.
Attempts have been made to overcome stem thread area corrosion issues, such as locating valve packing at a place between the liquid and the threads so that the threads are located on the exterior of the valve. An example is an OS & Y valve, the letter designation standing for outside screw and yoke. However, such solutions have created additional problems by exposing the stem threads to the surrounding environment, which can be problematic. For example, an inadvertent blow to the stem can cause bruising or bending of the stem, making the valve difficult or impossible to close. Additionally, dirt and other debris are attracted to lubricants that are used on the threads and this may result in difficulty operating the valve. Furthermore an OS & Y valve usually employs a bolted bonnet that must attach and seal to the body. The bolted bonnet requires an additional seal, which adds to manufacturing and maintenance costs.
Some instrument needle valves use the stem packing to isolate the stem threads from the internal fluid with an inside screw (instead of an outside screw) whereby the stem threads are located internal to the valve body. These valves employ a dual acting bonnet, which is threaded internally to accept the valve stem. Also, the bonnet may be axially adjusted to compress a packing ring. A drawback associated with such needle valves is that the needle valves employ a bonnet that is threaded externally to fit internal threads in the valve body. A locking mechanism, usually a jam nut, must be used to lock the bonnet into a fixed position with respect to the valve body after an adjustment to the compression packing has been made. Therefore, to tighten the packing, an operator must first loosen the jam nut. The loosening of the jam nut may not be obvious to an operator who normally tightens the packing on a leaking valve by one action only, i.e., tightening a packing nut. The non-intuitive aspect of first being required to loosen part of the valve before compressing of the packing is a disadvantage associated with this valve.
Another design weakness associated with using a rotating bonnet on a needle valve is revealed when a jam nut is left in a loosened condition, e.g., after a tightening sequence on the packing is performed. If the jam nut is left loose, then the next time an operator begins to unscrew the valve into a fully open position, a major diameter or flange that creates the back seat of the stem will rotate backwards until it bears against the inboard end of the bonnet. At this point, further backward rotation of the stem will cause the bonnet to unscrew from the valve body, which results in a possibly dangerous scenario that should be avoided. For this reason it is common to use a staking device or a secondary locking device to restrain movement of the bonnet. Use of a staking device or a secondary locking device is inconvenient because the device must first be removed before the packing can be tightened. Once such a device is removed the manufacturer has no assurance that it will be replaced.
Finally, another way to accomplish an isolation of the stem threads from fluid is to provide an O-ring seal between the diameter of the stem and the seal receiving area of the body. By using an O-ring around the stem instead of compression packing, the design may be greatly simplified because it is not necessary to provide a means of compressing the packing below the stem threads. However, this valve design is only as good as the seal. In a dirty fluid environment, such as oil and gas production, it is likely that a soft seal such as an elastomeric O-ring may become damaged by sharp angled debris such as rust, scale and sand. Axial and rotational movement of the stem as it travels between an open and closed position may pick up contaminants and drag them across the seal, causing cuts to the O-ring and consequent leakage.
In operation a certain amount of torque is required to turn a valve stem. The required torque creates the “feel” that a user experiences when opening or closing the hand wheel of the valve. Resistance to turning is caused by friction between the seal and the stem, by friction of the mating threads, and by hydraulic force internal to the valve body that acts against the stem.
If the valve utilizes an O-ring seal, then frictional resistance caused by the seal is typically low. If the valve utilizes compression packing for the seal then the seal friction is much greater and varies in proportion to the load imposed by a packing or stuffingbox nut. Some packing materials, such as Teflon®, will seal against the valve stem with relatively low loads but other materials, such as flexible graphite, require more significant stuffingbox loads.
As the valve stem advances into the valve body from an open position to a closed position, the stem rotates and travels axially through the seal area. The axial movement of the valve stem is resisted by internal hydraulic forces inside of the valve body. The force required to advance the stem is equal to the cross sectional area of the stem in the region of the packing run multiplied by the pressure. Thus, it can be seen that the force required to advance the stem against pressure can be lowered by making the seal diameter of the stem smaller for a given pressure condition. Therefore, it is advantageous to make the seal diameter of the stem as small as practical.
The valve stem has a threaded portion. The screw threads on the stem form an inclined plane and offer a mechanical advantage to overcoming the hydraulic force internal to the valve. A larger screw thread diameter produces a lower helix angle and therefore confers more advantage than a smaller thread diameter.
In a conventional valve the area of the stem between the threads and the hand wheel contains the stem seal. The area of the stem between the threads and the handwheel usually extends some distance past the outside of the packing or stuffingbox nut before it terminates at the hand wheel. The portion of the stem extending past the outside of the packing nut allows the handwheel to be separated from the rest of the valve, which permits the operator to grasp the hand wheel and turn it without scraping his knuckles on the valve body or adjacent piping. In the case of a gage valve of the type typically used with a liquid level gage, the hand wheel might be positioned not only over the valve body but also over a portion of the level gage. The handwheel should be positioned far enough from the valve to avoid interference between the hand wheel and the level gage. Thus, there must be sufficient extension of the stem to allow adequate clearance between the hand wheel and the valve for manual operation of the hand wheel. A second reason for an extended stem is to allow for placement of thermal insulation directly over the valve and still allow for operation of the hand wheel.
Since the extended stem is subject to damage, a manufacturer typically makes the stem of sufficient diameter to resist bending if the valve is dropped during installation or bumped sideways on the hand wheel during shipping, installation or operation. Additionally, a valve will sometimes “freeze” into position after a period of inactivity. A frozen valve often requires turning with a valve wrench. The valve wrench can impose severe torque on the hand wheel. If the stem diameter is too small then it can be twisted apart.